JP5044311B2 - Nitride single crystal growth method - Google Patents

Description

The present invention relates to a method for growing a nitride single crystal.

Gallium nitride III-V nitride has attracted attention as an excellent blue light emitting device, and has been put into practical use as a material for light emitting diodes and semiconductor laser diodes. In the methods described in Patent Documents 1 to 4, a group III nitride single crystal is grown by a flux method.
JP2002-293696A JP 2003-292400 A WO2005-095682 WO2006-030718

A method of controlling the nucleation site using a GaN thin film or an AlN thin film deposited on a substrate as a seed crystal has been reported (Patent Document 5).
JP 2000-327495 A

In addition, the present applicant decided to produce a needle-like aluminum nitride single crystal by a method for producing an aluminum nitride single crystal by heating a raw material composition containing alumina in a nitrogen-containing atmosphere in the presence of carbon. It has been successful (Patent Documents 6 and 7).
JP 2005-132699 A JP 2006-045047 A

In the flux method, the applicant decides to grow a single crystal uniformly on the seed crystal without generating miscellaneous crystals by providing a difference between the temperature at the gas-liquid interface and the temperature at the bottom of the melt. Successful (Patent Document 8: unpublished).
Japanese Patent Application No. 2006-084250

However, in the conventional method for growing a nitride single crystal, the nitride single crystal may be warped due to a difference in thermal expansion between the substrate and the nitride single crystal. That is, the grown nitride single crystal has a plate or film shape, but the nitride single crystal may be warped. Even when the nitride single crystal is warped, it is possible to form a flat surface by processing it. However, in a nitride single crystal in which warpage has occurred, the crystal orientation of the single crystal differs little by little depending on the location. For this reason, even if a flat surface is mechanically formed on the nitride single crystal, the crystal orientation at each point on the flat surface changes little by little. When the crystal orientation changes depending on the location, it becomes difficult to produce a high-quality optical device.
Further, when a nitride single crystal is grown on the surface of the seed crystal substrate, it is difficult to control the location of nucleation on the seed crystal substrate, and crystal growth starts at an arbitrary position on the seed crystal substrate. Nitride single crystals that have started to grow from adjacent sites collide with each other as they grow, and easily form grain boundaries.

  An object of the present invention is to suppress warping of crystal substrates and generation of grain boundaries when growing a nitride single crystal by a flux method.

The present invention is a method for growing a nitride single crystal by a flux method,
A nitride single crystal is grown from the side surface of a needle-like seed crystal composed of the nitride single crystal in the melt.

At this time, the c-axis of the growing nitride single crystal is made substantially parallel to the main axis of the acicular seed crystal, and the ratio of the length to the width of the acicular seed crystal is 5 or more.

A nitride single crystal substrate can be manufactured by processing the nitride single crystal.

  In the growing method of growing a nitride single crystal on a seed crystal in a melt, the present inventor increases the thickness or diameter of the needle-shaped seed crystal from the side surface in the radial direction, that is, the needle-shaped seed crystal. The idea was to grow nitride single crystals in the direction.

  For example, as shown in the schematic diagram of FIG. 1, the needle-like seed crystal 9 is immersed in the melt 5 of the growth vessel 7 to obtain a single crystal growth condition. According to this growth method, for example, as shown in the schematic diagrams of FIGS. 2A and 2B, nitriding is performed from the side surface 9a of the needle-like seed crystal 9 in the radial direction as indicated by the arrow Y. The material single crystal 10 grows one layer at a time. Therefore, collision between adjacent single crystals hardly occurs and a grain boundary is hardly formed.

  Moreover, since the single crystal obtained by this method has a small contact area with the acicular seed crystal, the influence of the difference in thermal expansion between the grown single crystal and the seed crystal is small. In addition, the shape of the single crystal is substantially symmetric around the needle-like seed crystal. Therefore, the single crystal to be grown is less warped, the crystal orientation of the single crystal is uniform, and a high-quality single crystal can be provided.

For example, the following single crystals can be suitably grown by the growing method of the present invention.
GaN, AlN, InN, mixed crystals thereof (AlGaInN), BN.

  The kind of the needle-like seed crystal is not particularly limited, but a nitride single crystal is particularly preferable. Examples of such nitride single crystals include GaN, AlN, InN, mixed crystals thereof (AlGaInN), and BN.

  The cross-sectional shape of the needle-like seed crystal is not particularly limited, and may be, for example, a perfect circle or an ellipse, or may be a polygon such as a triangle, a quadrilateral, a pentagon, or a hexagon.

The acicular seed crystals, Ri seed crystal der where the elongated shape, the length ratio to the width of the acicular seed crystals is 5 or more.

  Although the manufacturing method of an acicular seed crystal is not specifically limited, The method of patent document 6, 7 can be illustrated suitably.

  When the method of the present invention is carried out, the single crystal tends to grow from the side surface 9a toward the arrow A direction. Such a single crystal is also included in the present invention as long as its starting point is on the side surface 9a of the needle-like seed crystal 9. However, it is preferable that the single crystal grows from the side surface in the direction Y substantially perpendicular to the main axis X of the needle-like seed crystal 9.

  In a preferred embodiment, the nitride single crystal to be grown has a wurtzite structure, i.e., has an m-axis, a c-axis, and an a-axis. Each of these crystal axes is defined crystallographically. Then, the single crystal is grown so that the c-axis of the nitride single crystal to be grown is substantially parallel to the main axis of the acicular seed crystal. That is, as shown in FIGS. 2A and 2B, the nitride single crystal is grown so that the c-axis of the nitride single crystal 10 is substantially parallel to the main axis X of the needle-like seed crystal 9. As a result, warpage and grain boundaries of the grown nitride single crystal can be further reduced. In this embodiment, it is sufficient that the single crystal c-axis and the X-axis are substantially parallel. For example, an error within 10 ° is acceptable. Further, the orientation of the c-axis can be controlled by the crystal orientation on the side surface 9 a of the needle-like seed crystal 9.

  Further, a nitride single crystal obtained by the method of the present invention can be processed to produce a nitride single crystal substrate. This method is not particularly limited. For example, the nitride single crystal bulk can be ground, but it is preferable to cut the nitride single crystal bulk and take out the substrate. This cutting method is not particularly limited, and a normal cutting method for a single crystal boule can be used.

  In a preferred embodiment, the acicular seed crystal is cut so that the principal axis is parallel to the normal line of the nitride single crystal substrate. 3A and 3B relate to this embodiment.

  That is, as shown in the front view of FIG. 3A, the main axis X of the needle-like seed crystal 9 is cut out so as to be substantially parallel to the normal line K of the nitride single crystal substrate 12. T is a cut surface. The cut surface T is substantially perpendicular to the main axis X of the single crystal.

  By this cutting, a nitride single crystal substrate 12 as shown in FIG. 3B is obtained. 12a and 12b are main surfaces (cut surfaces). Dislocations extend in the growth direction of nitride single crystal 10 (that is, direction Y substantially perpendicular to main axis X). Therefore, when cutting in the direction Y substantially perpendicular to the main axis X, dislocations are not exposed on the main surfaces 12a and 12b of the substrate. Therefore, threading dislocations penetrating the substrate 12 can be reduced.

  In the practice of the present invention, the difference (TS−TB) between the temperature (TS) at the gas-liquid interface 5a of the melt 5 and the temperature (TB) at the bottom 5b of the melt 5 is 1 ° C. or more and 8 ° C. or less. (See FIG. 1). As a result, adhesion of miscellaneous crystals to the nitride single crystal can be prevented.

  In a preferred embodiment, the angle formed by the gas-liquid interface 5a of the melt 5 and the side surface 9a of the seed crystal 9 is set to 45 ° or more and 135 ° or less. Preferably, the angle formed by the gas-liquid interface 5a of the melt 5 and the side surface 9a of the seed crystal 9 is 80 ° or more and 100 ° or less. Particularly preferably, the gas-liquid interface 5a of the melt and the side surface 9a of the seed crystal are substantially vertical. This makes it difficult for the miscellaneous crystals to adhere to the single crystal.

Further, when the gas-liquid interface of the melt and the growth surface of the seed crystal are substantially perpendicular, a plurality of seed crystals can be accommodated and fixed in the growth vessel, so that productivity is improved.
The temperature of each point of the melt is measured by a thermocouple, a radiation thermometer, the amount of heat shrinkage of the ceramic molded body, and the like.

  In a preferred embodiment, as schematically shown in FIG. 4, a plurality of heating elements 6A, 6B, 6C are installed in the vertical direction, and the heating value is controlled independently for each heating element. That is, multi-zone control is performed in the vertical direction. It is generally difficult to control the temperature gradient in the vertical direction because the inside of the pressure vessel is high temperature and high pressure, but it is difficult to control the temperature gradient in the vertical direction. The temperature difference at can be optimally controlled.

  When each heating element is heated, a nitrogen-containing atmosphere is passed through the gas tank 1, the pressure control device 2, and the pipe 3 to the growth vessel 7 in the atmosphere control vessel 4, and when heated and pressurized, the mixed raw materials are mixed in the growth vessel. All dissolve and form a melt. Here, if the predetermined single crystal growth conditions are maintained, nitrogen is stably supplied into the growth raw material melt, and a single crystal film grows on the seed crystal.

  The material of the heating element is not particularly limited, but iron-chromium-aluminum alloy, nickel-chromium alloy heating element, platinum, molybdenum, tantalum, tungsten and other refractory metal heating elements, silicon carbide, molybdenum silicite, carbon Non-metallic heating elements such as

  In the single crystal growth apparatus, an apparatus for heating the raw material mixture to generate a melt is not particularly limited. This apparatus is preferably a hot isostatic pressing apparatus, but other atmospheric pressure heating furnaces may be used.

  The flux for generating the melt is not particularly limited, but one or more metals selected from the group consisting of alkali metals and alkaline earth metals or alloys thereof are preferable. Examples of the metal include lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium, and barium. Lithium, sodium, and calcium are particularly preferable, and sodium is most preferable.

Examples of the substance that forms an alloy with one or more metals selected from the group consisting of alkali metals and alkaline earth metals include the following metals.
Gallium, aluminum, indium, boron, zinc, silicon, tin, antimony, bismuth.

  The heating temperature and pressure in the single crystal growth step are not particularly limited because they are selected depending on the type of single crystal. The heating temperature can be set to, for example, 800 to 1500 ° C. Preferably it is 800-1200 degreeC, More preferably, it is 900-1100 degreeC. The pressure is not particularly limited, but the pressure is preferably 1 MPa or more, and more preferably 10 MPa or more. The upper limit of the pressure is not particularly defined, but can be, for example, 200 MPa or less, and preferably 100 MPa or less.

  The material of the growth container for carrying out the reaction is not particularly limited as long as the material is durable under the intended heating and pressurizing conditions. Examples of such materials include refractory metals such as tantalum, tungsten, and molybdenum, oxides such as alumina, sapphire, and yttria, nitride ceramics such as aluminum nitride, titanium nitride, zirconium nitride, and boron nitride, tungsten carbide, tantalum carbide, and the like. And pyrolytic products such as p-BN (pyrolytic BN) and p-Gr (pyrolytic graphite).

Hereinafter, more specific single crystals and their growth procedures will be exemplified.
(Gallium nitride single crystal growth example)
Using the present invention, a gallium nitride single crystal can be grown using a flux containing at least sodium metal. In this flux, the gallium source material is dissolved. As the gallium source material, a gallium simple metal, a gallium alloy, and a gallium compound can be applied, but a gallium simple metal is also preferable in terms of handling.

  This flux can contain metals other than sodium, such as lithium. The use ratio of the gallium source material and the flux source material such as sodium may be appropriate, but in general, it is considered to use an excess amount of sodium. Of course, this is not limiting.

  In this embodiment, a gallium nitride single crystal is grown under a pressure of a total pressure of 1 MPa or more and 200 MPa or less in an atmosphere composed of a mixed gas containing nitrogen gas. By setting the total pressure to 1 MPa or more, it was possible to grow a good quality gallium nitride single crystal in a high temperature region of, for example, 800 ° C. or higher, more preferably in a high temperature region of 900 ° C. or higher. The reason for this is not clear, but it is presumed that the nitrogen solubility increases as the temperature rises, and nitrogen is efficiently dissolved in the growth melt. In addition, if the total pressure of the atmosphere is 200 MPa or more, the density of the high-pressure gas and the density of the growth melt become quite close, so that it is difficult to hold the growth melt in the vessel for reaction. It is not preferable.

  In a preferred embodiment, the nitrogen partial pressure in the growth atmosphere is 1 MPa or more and 200 MPa or less. By setting the nitrogen partial pressure to 1 MPa or higher, for example, in a high temperature region of 800 ° C. or higher, dissolution of nitrogen into the flux was promoted, and a high-quality gallium nitride single crystal could be grown. From this viewpoint, it is more preferable that the nitrogen partial pressure of the atmosphere is 2 MPa or more. Moreover, it is preferable that nitrogen partial pressure shall be 100 Mpa or less practically.

A gas other than nitrogen in the atmosphere is not limited, but an inert gas is preferable, and argon, helium, and neon are particularly preferable. The partial pressure of a gas other than nitrogen is a value obtained by subtracting the nitrogen gas partial pressure from the total pressure.
In a preferred embodiment, the growth temperature of the gallium nitride single crystal is 800 ° C. or higher, preferably 900 ° C. or higher, and more preferably 1000 ° C. or higher. Even in such a high temperature region, a good quality gallium nitride single crystal can be grown. Moreover, there is a possibility that productivity can be improved by growing at high temperature and high pressure.

  The upper limit of the growth temperature of the gallium nitride single crystal is not particularly limited. However, if the growth temperature is too high, the crystal is difficult to grow. Therefore, the temperature is preferably set to 1500 ° C. or lower. From this viewpoint, the temperature is further set to 1200 ° C. or lower. preferable.

(Example of growing AlN single crystal)
The present invention has been confirmed to be effective even when growing an AlN single crystal by pressurizing a melt containing a flux containing at least aluminum and an alkaline earth in a nitrogen-containing atmosphere under specific conditions. .

Example 1
A GaN single crystal was grown in accordance with the present invention described with reference to FIGS.
Specifically, metallic gallium (Ga) as a group III raw material and metallic sodium (Na) as a flux were weighed in a growth vessel 7 together with a seed crystal. Weighed so that the molar ratio of Ga to Na was 25:75.

  As the seed crystal 9, a needle-like aluminum nitride single crystal having a diameter of 0.5 mm and a length of 10 mm was used. The single crystal 9 is manufactured according to Example 3 of JP 2006-045047 A. The seed crystal 9 was placed vertically and fixed so that the main axis X was perpendicular to the gas-liquid interface 5a. The growth container 7 was placed in an atmosphere control container 4 having a gas inlet and sealed. A series of operations were performed in an inert gas atmosphere in order to prevent oxidation of raw materials and flux.

  After the sealed container was placed in an electric furnace having three zones of heating elements 6A to 6C, the gas tank 1 was connected to the gas inlet through the pressure controller 2. The temperature of the three-zone heating elements can be individually adjusted, and an arbitrary temperature gradient can be provided in the vertical direction of the growth vessel. Nitrogen gas was introduced so as to have a gas pressure of 4 MPa in an atmosphere control vessel heated and maintained so that the temperature at the top of the growth vessel was 860 ° C. and the temperature at the bottom of the growth vessel was 855 ° C., and held for 200 hours. Thereafter, the growth vessel was taken out from the cooled atmosphere control vessel, and the flux was reacted with ethanol and removed, whereby the GaN single crystal grown around the seed crystal was collected.

  The grown GaN single crystal had a substantially hexagonal prism shape with a diameter of 8 mm and a length of 12 mm. There were no miscellaneous crystals on the single crystal.

  As shown in FIG. 3, the flat plate 12 was cut out from the grown GaN single crystal, and the surfaces 12a and 12b were polished to obtain a substrate having a thickness of 0.6 mm. The arithmetic mean roughness Ra of the substrate surface was estimated by atomic force microscopy. As a result, it was about 1 nm at a location 1 mm away from the needle-like seed crystal and about 1 nm at a location 1 mm inside from the outer periphery. Therefore, it was confirmed that the mirror polishing was almost uniformly performed within the surface. In addition, when the peak half-value width of the X-ray rocking curve was measured, the reflection from the (0002) plane was about 40 arcsec and the reflection from the (10-12) plane was about 30 arcsec. It could be confirmed. The roughness of the polished surface measured with a laser interferometer is 2 μm or less (measured from the number of interference fringes on the entire substrate using a He-Ne laser with an oscillation wavelength of λ = 633 nm), confirming that the warpage is small. did it.

  The substrate was immersed in an acidic solution (pH 3) at 50 ° C. for 3 hours, then washed, and the GaN portion was observed with a differential interference microscope. As a result, about 10 4 / cm 2 pits were observed on the c + plane. The c- plane was eroded violently and became uneven.

(Comparative Example 1)
As a seed crystal, a sapphire substrate having a thickness of (0001) and having a principal surface of (0001) mm was used without using a needle-like aluminum nitride single crystal. Thickness epitaxially grown on a sapphire substrate by metal organic chemical vapor deposition (MOCVD) 1
A GaN single crystal was grown in the same manner as in Example 1 except that a μm AlN single crystal thin film was used.

  The thickness of the recovered GaN single crystal could not be measured accurately due to surface irregularities, but was about 2 mm. When the sapphire surface side was scanned with a contact level meter, the radius of curvature of the substrate was about 2 m. The internal stress caused by the difference in thermal expansion coefficient between sapphire and GaN is considered to be the cause of such a large warp.

(Example 2)
The surface of the GaN single crystal substrate obtained in Example 1 was further polished to obtain a substrate having a thickness of 0.3 mm. The roughness of the polished surface measured using a laser interferometer in the same manner as in Example 1 was 2 μm or less. It was confirmed that the warpage of the substrate was small.

(Example 3)
As shown in FIG. 3, the flat plate 12 was cut out from the GaN single crystal obtained in Example 1, and the surface was polished to obtain a substrate 12 having a thickness of 1.0 mm. The roughness of the polished surface measured with a laser interferometer was 1 μm or less. It was confirmed that the warpage of the substrate was small.

Example 4
In the same manner as in Example 1, a GaN single crystal was obtained as shown in FIG. Next, as shown in FIG. 5B, the flat plate 13 shown in FIGS. 5B and 5C was cut out by cutting substantially parallel to the longitudinal direction of the needle-like single crystal 9. The main surfaces 13a and 13b of the flat plate 13 are (11-20) planes (a planes) in this example. The main surfaces 13a and 13b of the flat plate 13 were polished to obtain a substrate having a thickness of 0.6 mm. The unevenness of the polished surface measured with a laser interferometer was 3 μm or less, and it was confirmed that the warpage of the substrate was small. When the peak half-width of the X-ray rocking curve was measured, reflection from the (11-20) plane was about 30 arcsec, confirming that it was a high-quality single crystal.

(Example 5)
In the same manner as in Example 1, a GaN single crystal was obtained as shown in FIG. Next, as shown in FIG. 6A, the flat plate 14 shown in FIGS. 6A and 6B was cut out by cutting substantially parallel to the longitudinal direction of the needle-like single crystal 9. In this example, the main surfaces 14a and 14b of the flat plate 14 are (1-100) planes (m-planes). The main surfaces 14a and 14b were surface-polished to obtain a substrate 14 having a thickness of 0.6 mm. The unevenness of the polished surface measured with a laser interferometer was 3 μm or less, and it was confirmed that the warpage of the substrate was small. Further, when the peak half width of the X-ray rocking curve was measured, reflection from the (1-100) plane was about 30 arcsec, and it was confirmed that the crystal was a high quality single crystal.

FIG. 4 is a schematic view showing a state in which a melt 5 and a needle-like seed crystal 9 are accommodated in a growth container 7. (A) is a conceptual diagram which shows typically the state from which the single crystal is growing from the acicular seed crystal 9, (b) is a cross-sectional view similarly. (A) is a front view which shows the cutting method of a single crystal, (b) is a figure which shows the nitride single crystal substrate 12 cut | disconnected in (a). It is a block diagram which shows the whole growing apparatus which can be used for implementation of this invention. (A) is a front view schematically showing a grown single crystal, (b) is a top view showing a cutting direction of the single crystal of (a), and (c) is obtained by cutting. It is a front view which shows the flat plate 13. (A) is a top view which shows the cutting | disconnection direction of the single crystal of Fig.5 (a), (b) is a front view which shows the flat plate 14 obtained by cutting | disconnection.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 5 Melt 5a Gas-liquid interface 7 Growth container 9 Needle-shaped seed crystal 9a Side surface of needle-shaped seed crystal 9 10, 11 Nitride single crystal 12, 13, 14 Nitride single crystal substrate A Growth direction of nitride single crystal 11 C-axis of nitride single crystal 10 X main axis of needle-like seed crystal Y growth direction of nitride single crystal 10

Claims (6)

A method of growing a nitride single crystal by a flux method,
Nitrides from the side surface of acicular seed crystals of a nitride single crystal in the melt a single crystal is grown, as the c-axis of the nitride single crystal grown is substantially parallel to the main axis of the acicular seed crystals A method for growing a nitride single crystal , wherein the ratio of the length to the width of the needle-like seed crystal is 5 or more .   The difference (TS-TB) between the temperature (TS) at the gas-liquid interface of the melt and the temperature (TB) at the bottom of the melt is set to 1 ° C or more and 8 ° C or less. The method described.   The method according to claim 1, wherein the gas-liquid interface of the melt and the side surface of the seed crystal are substantially perpendicular. Characterized by producing a nitride single crystal substrate by processing the grown the nitride single crystal, the method according to any one of claims 1 to 3. In for machining the grown the nitride single crystal, wherein the acicular seed Succoth crystals spindle cut out the nitride single crystal substrate to be parallel to the normal line of the nitride single crystal substrate The method of claim 4 . The melt includes a gallium raw material and sodium, the needle-like seed crystal is made of an aluminum nitride single crystal, and the grown nitride single crystal is a gallium nitride single crystal. The method according to any one of claims 1 to 5.

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